A nail constituted by forming a head at an end of a shank having a pointed tip at an opposite end of the shank, the nail being generally called a round nail, is widely used as means for fixing, for example, wood materials to each other. There are also used a nail including a helical thread, which is called a screw nail, and a nail including ring-shaped projections formed on a shank, which is called a ring nail. In addition, threaded nails screwed into materials by employing a tool, such as an electrical screwdriver, are further used in many cases.
The round nail is driven into a material by hitting a nail head with a hammer, for example. Because the tip is pointed, the material such as a wood material, for example, into which the round nail is driven, is elastically deformed by the action of a wedge, and the shank is forced to thrust into the inside of the material. Hence the shank is fixedly held by elasticity of the material while undergoing gripping pressure. The round nail can be comparatively easily driven into woods or the likes. However, attachment strength of the round nail is given only by frictional resistance force under the gripping pressure due to the elasticity of the material, and the round nail is held in the material by frictional force. Therefore, the round nail has the problem that holding force is not so large and is apt to loosen. Furthermore, when pulling out the round nail, a special nail puller has to be used, and the surface of the material to which the round nail has been attached may be damaged.
The so-called screw nail includes a pointed tip in the form of a quadrangular pyramid, and a shank having an outer circumference along which helical ridges and grooves are formed in multiple threads. By hitting a head of the screw nail with a hammer, the screw nail is forced to thrust into woods, for example, while the shank is caused to rotate. Because the helical ridges and grooves are formed along the outer circumference of the shank, resistance against force in a pulling-out direction is large, and high holding force is obtained.
The so-called ring nail includes a pointed tip in the form of a quadrangular pyramid, and a shank having an outer circumference on which ring-shaped projections or recesses are formed. When the ring nail is driven into a material, the material is pushed to spread outward by outer circumferential portions of the rings. After being driven, when the ring nail is subjected to a pulling-out action, the outer circumferential portions of the rings come into, due to a restoration phenomenon of the material such as a wood material, a state caught by the restoring wood, and holding force is generated. The holding force is larger than that obtained with the round nail, but it is not so large.
On the other hand, the screw nail provides larger holding force than the round nail because, when the screw nail is driven into a material, it is forced to thrust into the material while rotating about a shank axis, and is fixedly held by engagement between a screw and the material. However, the screw nail has the problems that the screw nail is slightly pulled back even with a low load, and that there is a difficulty in entirely pulling out the screw nail and removing the attached material.
The threaded nail driven into a material by employing an electrical screwdriver, for example, is generally called a coarse thread. The coarse thread is the so-called half-thread screw that includes a head having a cross (+)-shaped recess formed to allow screwing by the electrical screwdriver, and a shank in which a portion of the entire length of the threaded nail closer to the head remains in the form of an element shank without being threaded, and in which a single-thread screw with a lead angle of about 20° is formed over the remaining shank portion from the above portion up to a tip. Thus, that type of threaded nail is intended to obtain higher attachment strength than the round nail, the screw nail, and the ring nail when used to attach wood materials to each other (see FIG. 6).
In trying to attach an attached material, which is a wood material or the like, to an underlying material, which is also a wood material or the like, with the above-mentioned type of threaded nail by employing the electrical screwdriver, for example, at a time when the threaded nail is screwed into the attached material from its surface and a tip of the threaded nail is just going to be screwed into the underlying material after penetrating through the attached material, the nail tip does not immediately come into the underlying material while the threaded nail advances relative to the attached material with continued rotation. Thus, due to the so-called time lag, the threaded nail is screwed into the underlying material in a state that a gap is generated between the underlying material and the attached material. To cope with the above-mentioned problem, the coarse thread is formed as the so-called half-thread screw without forming the thread over the entire length of the shank up to the head. Accordingly, it can be said that, even though high holding force relative to the underlying material is obtained with the threaded portion, the threaded portion is not present in a region corresponding to the attached material, and the attachment strength of the attached material with respect to the underlying material, which has been attached with the above-mentioned type of threaded nail, is at a level obtained with holding only by the driven head of the threaded nail.
Aiming to solve the above-described problems with the related-art nails, the inventor has developed an impact-driven threaded nail that includes a threaded portion formed in a shank, and has gained Japanese Patent (see Patent Literature (PTL) 1 denoted below). The patented impact-driven threaded nail has succeeded in significantly solving the problems with the related-art nails.